Page 308 - Trenchless Technology Piping Installation and Inspection

272 Cha pte r S i x

Based on these assumptions, and the preceding assumed values
for soil density and frictional coefficient, the estimated pull force
Eq. (6.3), leads to:

T (lb) = d (in.) × D (in.) × L (ft)/6 (6.5)
guide eff
where d is given by Eq. (6.4) and the conversion units for the
eff
various parameters are reflected in the “6” in the denominator of
Eq. (6.5).
For example, for the 100-ft-long, 5-ft-deep installation of the 4-in.
HDPE pipe considered in Sec. 6.10.2, for which Eq. (6.3) yielded a pull
force in excess of 17,500 lb, Eq. (6.5) predicts a required pull force on
the order of only 2000 lb, based on d of half the actual depth (for
eff
d/D = 60 in./4 in. = 15). Thus, this operation would appear to be well
within the capability of the DR 17 HDPE pipe under consideration.
As another example, consider a 250-ft section of 12-in. HDPE DR
17 pipe, placed with 10-ft cover. Based on Eq. (6.5), a required pull
force of approximately 30,000 lb is predicted, within the tabulated
safe pull strength of 32,500 lb. This result is consistent with field expe-
riences indicating a “routine” (see Table 6.1) operation for an installa-
tion of this approximation geometry.

6.10.4 Pipe Collapse Conditions
In addition to potential failure by excessive pulling tension, it is pos-
sible that the HDPE product pipe can significantly deform or collapse
during the installation phase or during the postinstallation (e.g.,
operational) stage. A relationship for critical (buckling) pressure, P ,
cr
is given in ASTM F1962-05 (ASTM, 2005) for unconstrained collapse
under uniform external (hydrostatic) pressure:

3
2
P = 2 Ef · f /{(1 – μ ) · (DR – 1) } (6.6)
cr o R
where E = material modulus of elasticity (psi)
μ = Poisson’s ratio (dimensionless)
f = ovality compensation (reduction) factor (dimensionless)
o
f = tensile stress reduction factor (dimensionless)
R
In a discussion of ASTM F1962-05, Petroff (2006) explains the
significance of these terms. The material properties, E and μ, for the
viscoelastic HDPE pipe depend on the load duration, f accounts for
o
initial or subsequent out-of-roundness, and f recognizes a potential
R
reduction in collapse strength in the presence of significant tensile
loads during the installation phase.
In the case of pipe bursting, and consistent with above discus-
sion, it must be anticipated that at least a portion of the expanded
bore path will be unstable and, therefore, tends to collapse with the
soil descending, applying vertical (and possibly a degree of lateral)